The removal of sulfur compounds from fossil hydrocarbon (HC) mixtures down to ppm levels is of major technical importance at various levels in industry and society due to the fact that sulfur compounds or derivatives thereof can have negative effects on technical operations and the environment. Legislation in the European community currently limits the sulfur level in fuels such as gasoline and diesel to 50 ppm and below.
The necessity for refineries to furnish ultra low sulfur fuels challenged established desulfurisation technologies, e.g. hydro desulfurisation (HDS), and lead to development of new deep desulfurisation (DDS) processes. The existing HDS technologies have a number of shortcomings in the application of DDS due to very high operating temperatures and pressures and, more importantly, the use of unsustainable large quantities of hydrogen.
New DDS processes comprise of contacting fuels after conventional desulfurisation (HDS, Merox, etc.) with a sulfur selective extractant and in many cases a supporting additive which are immiscible with the fuel phase.
Technologies other than HDS for the reduction of sulfur levels in HC fuels, include (i) oxidative desulfurisation (ODS) and (ii) extraction with ionic liquids. Both areas focus on the DDS of liquid HCs such as fuel oils, diesel fuel, jet fuel, gasoline, and crude with contents of ≦1500 ppm.
The area of ODS involves in the first step the oxidation of S-contaminants to sulfoxides and/or sulfones which exhibit low solubility in HCs, and are thus available for extraction into a suitable polar solvent in a subsequent step. Oxidants in this process typically consist of peroxides, in most cases aqueous hydrogen peroxide solutions. For example, several patents (U.S. Pat. Nos. 5,310,479; 6,402,940; EP 0565,324 A) and publications (T. Kabe et al. Energy & Fuels 2000, 14, 1232; Zannikos et al. Fuel Process Technol. 1995, 42, 35; T. Aida et al. Prep.-Am. Chem. Soc., Div. Pet. Chem. 1994, 39, 623; T. Hirai et al. Ind. Eng. Chem. Res. 2002, 41, 4362; Stournas et al. Fuel Process Technol. 1995, 42, 35; A. R. Lucy et al. J. Mol. Catal. A: Chemical 1997, 117, 397) disclose the application of organic carboxylic acids, e.g. formic acid, in conjunction with aqueous hydrogen peroxide.
Although these processes are effective they have a number of shortcomings namely: (i) the use of the peroxide oxidant in stoichiometric excess (2.5 to 3.5 times in U.S. Pat. No. 6,402,940; up to 1000 times in T. Hirai et al. Ind. Eng. Chem. Res. 2002, 41, 4362); (ii) the use of large amounts of flammable and volatile organic compounds; (iii) difficulty in rendering these processes both economically and environmentally benign; and (iv) occasional difficulties in recovering additives such as oxidiser/extractants; e.g. formic acid forms an azetrop when mixed with water which is difficult to break and requires additional process steps.
The second area of IL technology comprises of contacting ionic liquids with HCs such as diesel fuels, in which they are immiscible (US Pat Appl. 20050010076A1; Wasserscheid et al. Chem. Commun. 2001, 2494; Zng et al. Green Chem. 2002, 4, 376, U.S. Pat. No. 7,001,504 Schoonover). After gravity separation of the S-laden IL extractant and repeated extraction steps, model fuels with S-levels<50 ppm are obtained. A similar technology uses a combination of ILs and hydrogen peroxide as an oxidiser for the DDS of light oil (Wei et al. Green Chem. 2003, 5, 639).
Whilst ionic liquids have been known for many years, they have only recently attracted great interest as versatile materials due to their unique properties. They are defined as being liquids which consist of ions only and are also referred to as molten salts. Their attractive properties include, amongst others, a very low vapour pressure, good electrical conductivity, high chemical robustness and solubility characteristics which can easily be controlled by varying the nature of either the cation or anion (P. Wasserscheid, W. Keim Angew. Chem. 112 (2000) 3926; T. Welton, Chem. Rev. 99 (1999) 2071; J, d. Holbrey, K. R. Seddon, Clean Products and Processes, 1 (1999) 223).